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Concrete Technology

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Lecture Notes, UNIT 1, Cement: Portland cement – Chemical composition – Hydration, Setting of cement – Structure of hydrated cement – Test, on physical properties – Different grades of cement. Admixtures: Types of admixtures – mineral and chemical admixtures., , 1. Explain the Wet Process of manufacturing of Portland Cement., Ans:, The wet process is preferred when the raw materials are soft. The wet process of cement is manufactured by the following, procedure:, Mixing of Raw Materials:, 1. In the wet process, there are two raw materials i.e. calcareous and argillaceous. Initially, Calcareous materials are, crushed using crushers and argillaceous material is washed with water in the container., 2. After crushing, the line stones are stored in silos and after washing the clay is stored in basins., 3. The crushed materials from different silos and basins are drawn in correct proportions in a channel called wet, grinding mills. Both the materials are intimately mixed in the presence of water and to form a fine thin paste, known as slurry., 4. Slurry is then stored in another silo may be called as slurry silo where it is constantly stirred. The composition of, raw materials is checked again and, if required, corrected by adding clay or chalk materials as desires., Burning: In this operation, the slurry is directly fed into a long-inclined steel cylinder called a Rotary kiln. In this kiln there, are 3 different zones., (i) Drying Zones: In the wet process, the drying zone is comparatively larger than the dry process. It is because the raw, material in slurry form is directly fed into the kiln which has more amount of water. As shown in the figure it is the upper, portion of the kiln. In this zone, water is evaporated at a temperature of 100-400°C., (ii) Formations of modules: As the slurry gradually descends in the kiln, the carbon dioxide from the slurry evaporates and, small lumps formed which may be called modules., (iii) Burning Zone: The modules enter this zone where temperatures are kept about 1400-1500° C. The modules are, converted into dark greenish balls and the product obtained in the kiln, known as clinker, is of varying size 5 to 20 mm., The clinkers are very hot when coming out of this zone., (iv) Cooling of Clinkers: Another rotary kiln is provided in the opposite direction which is also inclined. It is used to cool, down the clinkers up to about 90°C., Grinding: The cooled clinkers are finely ground in ball mills or tube mills., Also, the gypsum is added during grinding about 2-4%. The gypsum acts as a retarder and so allows the cement to mix with, sand or aggregate and to be placed in position. i.e. it increases the initial setting time of cement., Storage and Packing: As cement comes out from grinding mills, it is collected in a hopper and taken in bucket elevator, for storage in silos. The cement from silos is packed by machines in bags. Each bag of cement contains 50 kg or 0.035, m3 of cement., 2. Explain the Dry Process of manufacturing of Portland Cement., Ans:, When the available raw materials are quite hard, then this process is used. The cement by this process can be prepared by, using the following operations:, a. Mixing of raw materials, b. Burning and Grinding, Mixing of Raw Materials: The raw materials i.e. argillaceous and calcareous materials undergo the following stages:, 1. Crushing: The raw materials, first of all, are broken into crushers into small fragments that vary in size., 2. Drying: The crushed materials are dried by heating at a sufficiently high temperature. It may be done in drying kilns., 3. Reduction of size: The drying materials are then ground by using ball mills and tube mills to reduce the size of materials, to find powder., Prepared by Kastro Kiran V, Email: [email protected]

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4. Mixing in correct proportion: The finely dried materials are mixed in exact proportions. The mixing may be done either, mechanically or by pneumatic methods (eg. pumped under pressure)., Burning, 1. The burning process is carried out in the rotary kiln while the raw materials are rotated at 1-2rpm at its longitudinal, axis., 2. As the kiln position is inclined and it rotates slowly, the material charged from upper end moves towards lower end at, the speed of 15m/hr., 3. In the upper part, water or moisture in the material is evaporated at 400 oC temperature, hence this zone is known as, Drying Zone., 4. The central part i.e. calcination zone, the temperature is around 10000C, where decomposition of lime stone takes place., The remaining material is in the form of small lumps known as nodules after the CO2 is released., 5. The lower part (clinkering zone) has temperature in between 1500-17000C where lime and clay react yielding calcium, aluminates and calcium silicates. These aluminates and silicates of calcium fuse to gather to form small and hard stones, known as clinkers. The size of the clinker is varying from 5-10 mm., 6. The clinker coming from the burning zone are very hot. To bring down the temperature of clinkers, air is admitted in, counter current direction at the base of the rotary kiln. The cooled clinkers are collected in small trolleys., Grinding of Clinkers, 1. The cooled clinkers are received from the cooling pans and sent into mills. The clinkers are grinded finely into powder, in ball mill or tube mill., 2. Powdered gypsum is added around 2-3% as retarding agent during final grinding. The final obtained product is cement., The grinded cement is stored in silos, from which it is marketed either in container load or 50kg bags., 3. Compare the advantages and disadvantages of wet and dry process of cement manufacturing (or) Differences, between Wet and Dry process of cement manufacturing., Ans:, Dry Process, Wet Process, The raw materials are mixed in dry state in blenders., The raw materials are mixed in wash mill with 35 to 50%, water., The dry materials exiting the mill are called “kiln feed”., The materials exiting the mill are called “slurry” and have, flowability characteristics., Fuel consumption is low in dry process i.e., 100 kg of coal Fuel consumption is high i.e., 350 kg of coal per tonne of, per tonne of cement produced., cement produced., Cost of production is less., Cost of production is high., Capital cost is high due to blenders., Capital cost (Cost of establishment) is comparatively less., Size of the kiln needed for manufacturing of cement is Size of the kiln needed for manufacturing of cement is, smaller., larger., Difficult to control mixing of Raw materials, so it is Raw material can be mixed easily, so a better homogeneous, difficult to obtain a better homogeneous material., material can be obtained., 4. Explain about different types of cement., Ans:, 1. Ordinary Portland Cement (OPC), Ordinary Portland cement is the most widely used type of cement which is suitable for all general concrete construction. It, is most widely produced and used type of cement around the world with annual global production of around 3.8 million, cubic meters per year. This cement is suitable for all type of concrete construction., 2. Portland Pozzolana Cement (PPC), Portland pozzolana cement is prepared by grinding pozzolanic clinker with Portland cement. It is also produced by adding, pozzolana with the addition of gypsum or calcium sulfate or by intimately and uniformly blending portland cement and fine, pozzolana., Prepared by Kastro Kiran V, Email: [email protected]

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This cement has high resistance to various chemical attacks on concrete compared with ordinary portland cement and thus, it is widely used. It is used in marine structures, sewage works, sewage works and for laying concrete under water such as, bridges, piers, dams and mass concrete works etc., 3. Rapid Hardening Cement, Rapid hardening cement attains high strength in early days it is used in concrete where formworks are removed at an early, stage and is similar to ordinary portland cement (OPC). This cement has increased lime content and contains higher c3s, content and finer grinding which gives greater strength development than OPC at an early stage., The strength of rapid hardening cement at the 3 days is similar to 7 days strength of OPC with the same water-cement ratio., Thus, advantage of this cement is that formwork can be removed earlier which increases the rate of construction and, decreases cost of construction by saving formwork cost. Rapid hardening cement is used in prefabricated concrete, construction, road works, etc., 4. Quick setting cement, The difference between the quick setting cement and rapid hardening cement is that quick setting cement sets earlier while, rate of gain of strength is similar to Ordinary Portland Cement, while rapid hardening cement gains strength quickly., Formworks in both cases can be removed earlier. Quick setting cement is used where works is to be completed in very short, period and for concreting in static or running water., 5. Low Heat Cement, Low heat cement is prepared by maintaining the percentage of tricalcium aluminate below 6% by increasing the proportion, of C2S. This makes the concrete to produce low heat of hydration and thus is used in mass concrete construction like gravity, dams, as the low heat of hydration prevents the cracking of concrete due to heat. This cement has increased power against, sulphates and is less reactive and initial setting time is greater than OPC., 6. Sulphates Resisting Cement, Sulfate resisting cement is used to reduce the risk of sulphate attack on concrete and thus is used in construction of, foundations where soil has high sulphate content. This cement has reduced contents of C3A and C4AF. Sulfate resisting, cement is used in construction exposed to severe sulphate action by water and soil in places like canals linings, culverts,, retaining walls, siphons etc., 7. High Alumina Cement, High alumina cement is obtained by melting mixture of bauxite and lime and grinding with the clinker. It is a rapid, hardening cement with initial and final setting time of about 3.5 and 5 hours respectively., The compressive strength of this cement is very high and more workable than ordinary portland cement and is used in, works where concrete is subjected to high temperatures, frost, and acidic action., 8. White Cement, It is prepared from raw materials free from Iron oxide and is a type of ordinary portland cement which is white in color. It, is costlier and is used for architectural purposes such as precast curtain wall and facing panels, terrazzo surface etc. and for, interior and exterior decorative work like external renderings of buildings, facing slabs, floorings, ornamental concrete, products, paths of gardens, swimming pools etc., 9. Colored cement, It is produced by mixing 5- 10% mineral pigments with ordinary cement. They are widely used for decorative works in, floors., 10. Air Entraining Cement, Air entraining cement is produced by adding indigenous air entraining agents such as resins, glues, sodium salts of sulphates, etc. during the grinding of clinker., This type of cement is especially suited to improve the workability with smaller water cement ratio and to improve frost, resistance of concrete., 5. Write in brief about Hydration and Heat of Hydration of cement., Ans:, Hydration of cement: The chemical reaction that takes place between cement and water is known as hydration of cement., Anhydrous cement compounds when mixed with water reacts each other to form hydrated cement compounds of very low, solubility. The hydration of cement can be visualized in two ways. Primary is ‘through solution.’ In this, the cement, Prepared by Kastro Kiran V, Email: [email protected]

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compounds dissolve to produce a super saturated solution from which different hydrated products gets precipitated., Secondary is ‘solid state.’ In this the water attacks the cement compounds in the solid state converting the compounds into, hydrated products starting from the surface and proceeding to the interior of the compounds with time., Heat of Hydration: The reaction of cement with water is exothermic. This reaction liberates considerable amount of heat, and hence this liberation of heat is called heat of hydration. The heat of hydration is the quantity of heat, in joules per gram, of unhydrated cement, evolved upon complete hydration at a given temperature. The actual value of heat of hydration, depends on the chemical composition of the cement., 6. Explain the importance of Bogue’s compounds (or) Explain the functions of Bogue’s compounds (or) Explain, the composition of Cement clinker., Ans:, The raw materials used for the manufacture of cement are Lime, Silica, Alumina and Iron which react with oxide, compound., These oxides interact with one another in the kiln at high temperature to form more complex compounds. The identification, of these compounds is majorly based on R.H. Bogue’s work and hence they are called as ‘Bogue’s Compounds.’, The change in proportions of these oxide compositions is responsible for influencing the various properties of cement. The, various constituents combine in burning and form cement clinker. The compounds formed in the burning process have the, properties of setting and hardening in the presence of water., 1) Tricalcium Silicate – C3S, a) The chemical formula of Tricalcium Silicate is 3CaO.SiO2. It is also called as Alite. In cement Alite compound is, about 45-60%., b) Alite is responsible for early strength (7 days)., c) It liberates high amounts of heat during the early stages of hydration. On cooling below 2500C, it decomposes., d) C3S hardens rapidly and require 24% of water., e) Tricalcium silicates appears in Light Grey Crystals when seen under microscope and size of Alite ranges between, 20-50 micro meter., f) Cement with more C3S content is better for cold weather concreting. The heat of hydration is 500 J/g., 2) Dicalcium Silicate – C2S, a) The chemical formula of Dicalcium Silicate is 2CaO.SiO2. It is also called as Blite. In cement Blite compound is, about 15-30%., b) Belite is responsible for later strength i.e strength after 7 days., c) It produces less heat of hydration and hence Belite hydrates and hardens slowly., d) Belite requires about 21% of water and it shows resistance to chemical attack., e) It appears as Dark Grey Crystals when seen under microscope and size of Belite ranges between 15-20 micrometer., f) Alite and Belite together forms Calcium Silicate Hydrate – CSH (C3S2H3). The heat of hydration is 260 J/g., 3) Tricalcium Aluminate – C3A, a) The chemical formula of Tricalcium Aluminate is 3CaO.Al2O3. It is also called as Celite. In cement Celite, compound is about 6-12%., b) The reaction of C3A with water is very fast and causes initial setting of cement and leading to immediate stiffening, of paste known as flash set., c) Addition of 2-3% gypsum regulates the setting time. Celite doesn’t contribute to strength., d) Celite requires about 40% of water and high amounts of heat., e) It appears as Small dark inclusions when seen under microscope. The heat of hydration of 865 J/g., 4) Tetracalcium Alumino Ferrite – C4AF, a) The chemical formula of Tetracalcium Alumino Ferrite is 3CaO.Al2O3Fe2O3. It is also called as Felite. In cement, Felite compound is about 6-8%., b) Felite imparts colour to the cement and appears as Bright Interstitial material., Felite is more stable than C3A and helps in fusion of raw materials. The heat of hydration of 420p J/g., , Prepared by Kastro Kiran V, Email: [email protected]

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7. Explain the heat evolution Pattern in Cement hydration (or) Explain the stages in Cement hydration., Ans:, The chemical reaction that takes place between cement and water is known as Hydration of cement. Hydration is series of, chemical reactions of cement with water to form the binding material (C-S- H Gel)., The pattern of heat evolution during the hydration of cement is visualized in five phases., Phase 1: Initial mixing reaction, Phase 2: Dormancy, Phase 3: Strength acceleration, Phase 4: Speed reduction, Phase 5: Steady development, Phase 1: Initial mixing reaction, a) When cement and water come into contact with each other, a peak in temperature happens., b) The aluminate (C3A) reacts with H2O (calcium and sulfate ions) to form ettringite (aluminate hydrate). The release, of the energy from these reactions causes the initial peak., Phase 2: Dormancy, a) A result of the reaction described in Phase 1 is a surface coating of the cement particles., b) This coating keeps increases, but also slows down the hydration reaction as the access to water isn’t as good as, when the concrete was mixed., c) The concrete in this phase is used for transporting and pouring, as the concrete stays on a fluid level., d) The length of this period depends on each individual concrete mix., e) This phase ends with an initial set of the concrete., Phase 3: Strength acceleration, a) A major heat increase happens due to the reaction between calcium-silicate (C3S and C2S) which creates the silicate, hydrate CSH., b) The creation of CSH also has a major impact on the concrete strength during this phase., c) In the case of mass concrete application, it can be very important to monitor the internal temperature variances, as, the concrete temperature during this phase can increase rapidly to reach internal temperatures like 70-80C., d) It is normally not recommended to exceed temperatures at around 70C., Phase 4: Speed reduction, a) A maximum temperature has now been reached and the availability of free particles is now reduced and therefore, slows down the temperature increase., b) This phase often ends with the desired strength and the formwork around the concrete can now be removed., c) Monitoring of concrete maturity and temperature and therefore enable the user with the exact time where this is, possible., Phase 5: Steady development, a) The hydration process is now slowed down and will continue slowly to finish the remaining available cement and, water particles., b) The formwork is now often removed and the concrete will now over time finish the hydration process and reach, final strengths which can take weeks or months., , Prepared by Kastro Kiran V, Email: [email protected]

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8. Explain the functions of Lime, Silica, Alumina, Iron oxides, Magnesium and Sulphur trioxide in cement., Ans:, 1. Functions of Lime in Cement, a) It is the main constituent used in cement manufacturing. It is responsible for imparting the cementing property to, cement., b) An excess quantity of lime causes expansion and disintegration of the cement. Deficiency of lime causes decreases, strength and the cement sets quickly., c) If it is used in the right proportion, it makes the cement sound and strong. Thus, lime can drastically change, the properties of cement., 2. Functions of Silica in Cement, a) This plays a major role in imparting strength to concrete., b) Silica undergoes a chemical reaction with calcium to form dicalcium silicate (C2S) and tricalcium silicates (C3S)., c) Excess silica adds strength to cement but it prolongs the setting time., 3. Functions of Alumina in Cement, a) This forms complex compounds with silica and calcium to improve the setting capacity of the cement. It acts as a, flux and lowers the clinkering temperature., b) Use of an excess amount of alumina quickens the setting time but reduces the strength of cement., 4. Functions of Ferrous Oxide in Cement, a) Ferrous oxide imparts grey color to the cement in addition to the hardness and strength., b) It also helps in the fusion of raw materials during the manufacture of cement., 5. Function of Magnesium Oxide in cement, a) It imparts strength to the cement when mixed in a small quantity; however, an excess amount makes the cement, unsound., 6. Function of Sulphur Trioxide in cement, a) This makes the cement sound if present in small quantity but an excess can make the cement unsound., 9. Explain about the physical and chemical properties of cement., Ans:, Physical properties of cement, 1. Fineness: The quality of Cement depends upon its fineness property. So good Cement should be fine. It is a measure of, size of particles of cement. The finer the cement the higher the rate of hydration, as more surface area is available for, chemical reaction. For a given weight, the surface area is more for finer cement than coarser cement. Fineness if typically, measured using the Blaine air permeability test, and is of the order of 300 – 400 m2/kg for OPC., 2. Consistency: The normal consistency and setting time are determined using the Vicat apparatus. Normal consistency is, an empirical measure that indicates the minimum water required to produce a certain level of fluidity in the cement paste., It also enables the design of the paste for the setting time and compressive strength experiments. The initial setting time is, , Prepared by Kastro Kiran V, Email: [email protected]

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important to assess the time available for concreting operations (transportation, placement, consolidation, and finishing),, while the final setting time indicates the attainment of a specific form (i.e. concrete beyond this point cannot be remoulded)., 3. Soundness: The soundness of cement is related to its ability to retain its volume upon hydration, and is measured using, the Le Chatelier apparatus. All types of cement expand to some extent and it depends upon the composition, proper burning,, and grinding of cement. A good quality Cement should not undergo a large change in volume after setting. Also, it should, not crack or disintegrate after setting. Soundness is due to excess lime., 4. Strength: The compressive strength of cement is measured in mortar because of the poor dimensional stability of the, cement paste (high shrinkage), as well as the fact the by using mortars, one can actually check the ‘binding’ strength of, cement paste, which reflects its true action in concrete. The codes have strict requirements not only in terms of the 28 day, strength, but also for the intermediate strengths (3 and 7 days). For OPC the compressive strength at 3 &7 days shall not be, less than 16 MPa and 22 MPa., 5. Heat of hydration: 120 cal/g heat will be liberated, 6. Specific gravity: The specific gravity of cement can be measured using a LeChatlier’s flask, using an inert liquid such, as oil or kerosene. Typical values are in the range of 3.10 – 3.16. However, when packed as bulk powder, the specific, gravity of cement is only 1.5 – 1.6., Chemical Properties of cement, 1. Loss on ignition test: To find loss of weight. Sample is heated at 900-10000C for 15minutes minimum. Maximum, allowable loss is 4%., 2. Insoluble residue: Determined by stirring 1g of cement in 40ml of water and adding 10ml of concentrated HCl/ mix is, boiled for minimum 10min maintaining a constant volume. The solution is then filtered. The residue s washed with Na2CO3, solution, water and HCl and again with water. Filter paper is dried, ignited and weighed. Minimum is the residue, better is, the cement. 0.85% is the limit., 10. Explain about the field tests that are to be performed on cement., Ans: The different field tests that are performed on cement are Colour test, Presence of lumps test, Adulteration test,, Temperature Test of Cement, Float test, Setting time test, Strength of cement test and Date of packaging., 1. Colour test:, a) The typical colour of the cement should be grey with a light greenish side., b) The colour of the cement should be uniform., 2. Presence of lumps test:, a) Lumps are formed when cement comes in contact with moisture in the atmosphere., b) Cement with lumps is not advised for construction., c) Any bag which is delivered to the site with lumps should be rejected., 3. Adulteration test:, a) This test is performed by rubbing the cement with fingers., b) Cement should feel smooth when rubbed in between fingers. If it feels rough, it implies that the cement is, adulterated with sand., 4. Temperature test:, a) This test involves inserting the hand in the bag to check its temperature. It should be cool inside., b) If it’s warm inside, it indicates that the process of hydration is taking place., c) It is better to store and cool the cement before using it., 5. Float test:, a) This test consists of throwing some handful of cement in the bucket full of water., b) The cement particles should float for some time before sinking., 6. Setting time test:, a) In this test, a thick paste is prepared by mixing cement and water and is placed on a piece of glass plate. This is, kept under water for 24hours. After this, cement should set and not crack., , Prepared by Kastro Kiran V, Email: [email protected]

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7. Strength of cement test:, a) This test involves preparation of cement block of dimension 25 mm x 25 mm and length of 200 mm. It is, immersed for 7 days in water. It is then placed on supports 150 mm apart and it is loaded with a weight of about, 34kg. If the block will not show any failure, then the cement is of good quality., 11. Explain the chemical reactions involved in the process of hydration., Ans:, When water is added to cement, the following series of reactions occur:, ▣ Tricalcium aluminate + gypsum + water ® ettringite + heat (207 cal/g), C3A + 3CSH2 + 26H, C6AS3H32, D H = 207 cal/g, Ettringite - long crystals that are only stable in a solution with gypsum. The compound does not contribute to the strength, of the cement glue., ▣ Tricalcium silicate + water ® calcium silicate hydrate + lime + heat (120 cal/g), 2C3S + 6H, C3S2H3 + 3CH, D H = 120 cal/g, The CSH has a short-networked fiber structure which contributes greatly to the initial strength of the cement glue., Once all the gypsum is used up as per reaction (i), the ettringite becomes unstable and reacts with any remaining tricalcium, aluminate to form monosulfate aluminate hydrate crystals:, ▣ Tricalcium aluminate + ettringite + water ® monosulfate aluminate hydrate, 2C3A + 3 C6AS3H32 + 22H, 3C4ASH18, The monosulfate crystals are only stable in a sulfate deficient solution. In the presence of sulfates, the crystals resort back, into ettringite, whose crystals are two-and-a-half times the size of the monosulfate., It is this increase in size that causes cracking when cement is subjected to sulfate attack., The ferrite undergoes two progressive reactions with the gypsum:, in the first of the reactions, the ettringite reacts with the gypsum and water to form ettringite, lime and alumina hydroxides,, i.e., ▣ Ferrite + gypsum + water ® ettringite + ferric aluminum hydroxide + lime, C4AF + 3CSH2 + 3H, C6(A,F)S3H32 + (A,F)H3 + CH, the ferrite further reacts with the ettringite formed above to produce garnets, i.e., ▣ Ferrite + ettringite + lime + water ® garnets, C4AF + C6(A,F)S3H32 + 2CH +23H, 3C4(A,F)SH18 + (A,F)H3, The garnets only take up space and do not, in any way, contribute to the strength of the cement paste., The belite (dicalcium silicate) also hydrates to form calcium silicate hydrates and heat:, ▣ Dicalcium silicates + water ® calcium silicate hydrate + lime, C2S + 4H, C3S2H3 + CH, D H = 62 cal/g, Like in reaction (ii), the calcium silicate hydrates contribute to the strength of the cement paste., This reaction generates less heat and proceeds at a slower rate, meaning that the contribution of C 2S to the strength of the, cement paste will be slow initially. This compound is however responsible for the long-term strength of Portland cement, concrete., 12. Explain about the hydrated products of cement hydration., Ans:, The products of the reaction between cement and water are termed as "hydration products." In concrete (or mortar or other, cementitious, materials), hydrated, products, are, typically, of, four, main, types:, Calcium Silicate Hydrate: This is the main reaction product and is the main source of concrete strength. It is often, abbreviated, using cement chemists' notation, to "C-S-H," the dashes indicating that no strict ratio of SiO2 to CaO is, inferred. The Si/Ca ratio is somewhat variable but typically approximately 0.45-0.50 in hydrated Portland cement but up to, perhaps about 0.6 if slag or flyash or micro-silica is present, depending on the proportions., Calcium Hydroxide: It is also called as Portlandite. Ca(OH)2 often abbreviated to 'CH.' CH is formed mainly from alite, Prepared by Kastro Kiran V, Email: [email protected]

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hydration. Alite has a Ca:Si ratio of 3:1 and C-S-H has a Ca/Si ratio of approximately 2:1, so excess lime is available to, produceCH., AFm and AFt phases: These are two groups of minerals that occur in cement, and elsewhere. One of the most common, AFm phases in hydrated cement is monosulfate and the most common AFt phase is ettringite. Ettringite is an AFt phase, because it contains three (t-tri) molecules of anhydrite when written as C3A.3CaSO4.32H2O and monosulfate is an AFm, phase because it contains one (m-mono) molecule of anhydrite when written as C3A.CaSO4.12H2O., The, most, common, AFt, and, AFm, phases, in, hydrated, cement, are:, Ettringite: Ettringite is present as rod-like crystals in the early stages of reaction or sometimes as massive growths filling, pores or cracks in mature concrete or mortar. The chemical formula for ettringite is [Ca3Al(OH)6.12H2O 2.2H2O] or, mixing, notations,, C3A.3CaSO4.32H2O., Monosulfate: Monosulfate tends to occur in the later stages of hydration, a day or two after mixing. The chemical formula, for monosulfate is C3A.CaSO4.12H2O. Note that both ettringite and monosulfate are compounds of C3A,, CaSO4 (anhydrite), and, water,, in, different, proportions., Monocarbonate: The presence of fine limestone, whether interground with the cement or present as fine limestone, aggregate, is likely to produce monocarbonate (C3A.CaCO3.11H2O) as some of the limestone reacts with the cement pore, fluid., 13. Discuss in brief about the raw materials and chemical composition of cement., Ans:, The raw materials used for the manufacturing of Portland cement are found naturally in the earth’s crust. Cement is, primarily made from calcareous and argillaceous materials and gypsum. Calcareous materials contain limestone or chalk, while argillaceous materials comprise an oxide of silica-alumina and iron., The chemical composition of cement is as follows:, , 14. Explain the structure of Hydrated cement., Ans:, a) Hydrated cement paste is composed of capillary pores and the hydration products., b) The pores within the structure of the hydration product are termed as ‘gel’ pores., c) The hydration products includes C-S-H, CH, AFt, AFm, etc. Gel pores are included within the structure of hydrated, cement., d) According to Power’s, 1/3rd of the pore space is comprised of gel pores, and the rest are capillary pores., e) The pores inside cement paste contain water (or pore solution), which can be classified into:, 1. Capillary water: Present in voids larger than 50 Ao. Further classified into: (a) free water, the removal of which, does not cause any shrinkage strains, and (b) water held by capillary tension in small pores, which causes shrinkage, strains on drying., 2. Adsorbed water: Water adsorbed on the surface of hydration products, primarily C-S-H. Water can be physically, adsorbed in many layers, but the drying of farther surfaces can occur at about 30 % relative humidity. Drying of, this water is responsible for a lot of shrinkage., Prepared by Kastro Kiran V, Email: [email protected]

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3. Interlayer water: Water held in between layers of C-S-H. The drying of this water leads to a lot of shrinkage due, to the collapse of the C-S-H structure., 4. Bound water: This is chemically bound to the hydration product, and can only be removed on ignition. Also called, ‘non-evaporable’ water., Adsorbed water and Interlayer water are called as ‘gel’ water., , f) The structure of C-S-H is best described by the Feldman-Sereda model, shown in the above figure. It consists of, randomly oriented sheets of C-S-H, with water adsorbed on the surface of the sheets (adsorbed water), as well as in, between the layers (interlayer water), and in the spaces inside (capillary water)., g) Calcium hydroxide deposits as hexagonal crystals. These crystals are typically aligned in the long direction inside pores, and around aggregate surfaces., h) The structure of ettringite consists of tubular columns with channels in between the columns. The imbibing of water in, these channels can lead to substantial expansions. Ettringite demonstrates a trigonal structure, while monosulfate is, monoclinic., , Note:, The laboratory tests on cement are not included in this material, it is advised to refer, Concrete Technology by MS Shetty textbook for more information on test procedures., , Prepared by Kastro Kiran V, Email: [email protected]